A TDS scale error is not a broken pen — it is a mismatched multiplier. PPM is a calculated number, not a measurement: EC is the only true reading, and the factor the firmware baked in — 500 NaCl, 640 KCl or 700 USA 442 — decides what the display shows. The same 1.8 mS/cm solution reads 900 ppm on the 500 scale, 1,152 ppm on the 640 scale and 1,260 ppm on the 700 scale, and all three are correct. The error appears only when a reading from one scale is compared against a chart written for another.
Key takeaways
- Punch in EC and it returns PPM on the 500, 640 and 700 scales, plus CF and µS/cm, so any meter and any nutrient chart finally agree.
- A 1.8 mS/cm solution reads 900 ppm on the 500 scale, 1260 ppm on the 700 scale, and both are correct.
- EC drifts with solution temperature (~2%/°C).
- Calibrate in 1413 µS/cm buffer and let the reservoir settle before judging a feed.
Why the TDS Meter Scale Error Appears: 500, 640 and CF
- Enter the meter reading as EC in mS/cm — the only number you should actually trust from a conductivity pen.
- Select the TDS scale printed on your meter: 500 NaCl, 640 KCl or 700 (USA 442). The hub converts all three every time.
- Read PPM on your chosen scale plus CF and µS/cm simultaneously so any two pieces of gear can be reconciled.
- Compare any two nutrient formulas element-by-element at the same EC to see where one pulls ahead.
- For pH, enter current and target pH plus tank volume; the hub suggests the pH Down / pH Up dose for the medium.
- Retest 30 minutes after any pH or feed change — the dose is a starting point, never the final word.
The Formula Behind the Result
TDS on the USA 442 scale → text{PPM}_{700} = text{EC} ,(text{mS/cm}) times 700NaCl scale — typical of £/American hand-held meters → text{PPM}_{500} = text{EC} times 500EC Factor — the Commonwealth grower scale → text{CF} = text{EC} times 10Conductivity without the milli prefix → text{µS/cm} = text{mS/cm} times 1000pH up/down dose for the tank volume and buffering factor → text{mL} = V,(text{L}) times |Delta pH| times k_{medium}Elemental delta between two formulas at the running EC → Delta e_{i} = k_{i}^{A},text{EC} - k_{i}^{B},text{EC}
Target Ranges: What the Numbers Should Be
- PPM is a calculation the meter makes, not a physical measurement. Only EC is truly measured; the multiplier is a style choice your meter’s firmware baked in.
- Always match the scale to your nutrient charts. A 1.8 mS/cm solution reads 900 ppm on the 500 scale, 1260 ppm on the 700 scale, and both are correct.
- EC drifts with solution temperature (~2%/°C). Calibrate in 1413 µS/cm buffer and let the reservoir settle before judging a feed.
- pH Down liquid should be diluted 1:10 before dosing small tanks, and always distributed around the reservoir, never dumped in one spot.
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Related guides
Worked example: 1.8 mS/cm read on three different scales
Calibrate the pen in 1413 µS/cm buffer, let the reservoir settle, and take the reading at 1.8 mS/cm. On the 500 scale (NaCl hand-held meters): 1.8 × 500 = 900 ppm. On the 640 scale (KCl): 1.8 × 640 = 1,152 ppm. On the 700 USA 442 scale: 1.8 × 700 = 1,260 ppm. CF is 1.8 × 10 = 18, and µS/cm is 1,800. Every figure describes the same solution. The scale error is comparing 1,260 ppm (700 scale) against a 900–1,100 ppm chart written for the 500 scale — it looks like overfeeding when it is not. Temperature adds its own drift at roughly 2% per °C, so a reservoir 5 °C warm inflates a 1.8 mS/cm reading by about 10%.
- 1.8 mS/cm × 500 (NaCl) = 900 ppm.
- 1.8 mS/cm × 640 (KCl) = 1,152 ppm.
- 1.8 mS/cm × 700 (USA 442) = 1,260 ppm.
- CF = 18, µS/cm = 1,800 — match the scale to the chart, never mix.
Frequently Asked Questions
References & Science
- Cornell CALS — hydroponic research manual
- NCSU CALS — Vapor Pressure Deficit resources
- USDA National Agricultural Library
Frequently asked questions
Why does my TDS meter show a different number than the chart?
Because the meter and the chart are written on different scales. EC is the only true measurement; PPM is EC multiplied by the firmware’s factor — 500, 640 or 700. Read EC in mS/cm from a calibrated pen, then convert to the PPM scale your nutrient chart actually uses before comparing.
What is the biggest mistake behind a TDS scale error?
Comparing a PPM reading against a chart calibrated on another scale, and judging a feed from an unsettled or uncalibrated reservoir. Calibrate in 1413 µS/cm buffer, let the solution settle, and match the scale before adding anything.
Which tool helps me convert between TDS scales?
The EC / PPM / CF converter on this page returns all three scales, CF and µS/cm from a single EC input. Use it before dosing, and never judge a feed until the reservoir has settled and the scale matches the chart.
References
- Resh, H. M. — Hydroponic Food Production (7th ed.), CRC Press
- ISHS Acta Horticulturae — nutrient film technique (NFT) system publications
- Cooper, A. (1979) — The ABC of NFT, Grower Books
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